Problem: Simplify the following expression and state the condition under which the simplification is valid. $z = \dfrac{t^2 - 9}{t + 3}$
Explanation: First factor the polynomial in the numerator. The numerator is in the form ${a^2} - {b^2}$ , which is a difference of two squares so we can factor it as $({a} + {b})({a} - {b})$ $ a = t$ $ b = \sqrt{9} = 3$ So we can rewrite the expression as: $z = \dfrac{({t} + {3})({t} {-3})} {t + 3} $ We can divide the numerator and denominator by $(t + 3)$ on condition that $t \neq -3$ Therefore $z = t - 3; t \neq -3$